A computing system may use an interface to connect to one or more peripheral devices, such as data storage devices, printers, and scanners. The interface typically includes a data communication bus that attaches and allows orderly communication among the devices and the computing system. A system may include one or more communication buses. In many systems a logic chip, known as a bus controller, monitors and manages data transmission between the computing system and the peripheral devices by prioritizing the order and the manner of device control and access to the communication buses. Control rules, also known as communication protocols, are imposed to promote the communication of information between computing systems and peripheral devices. For example, Small Computer System Interface or SCSI (pronounced “scuzzy”) is an interface, widely used in computing systems, such as desktop and mainframe computers, that enables connection of multiple peripheral devices to a computing system.
In a desktop computer SCSI enables peripheral devices, such as scanners, CDs, DVDs, and Zip drives, as well as hard drives to be added to one SCSI cable chain. In network servers SCSI connects multiple hard drives in a fault-tolerant cluster configuration in which failure of one drive can be remedied by replacement from the SCSI bus without loss of data while the system remains operational. A fault-tolerant communication system detects faults, such as power interruption or removal or insertion of peripherals, allowing reset of appropriate system components to retransmit any lost data.
A SCSI communication bus follows the SCSI communication protocol, generally implemented using a 50 conductor flat ribbon or round bundle cable of characteristic impedance of 100 Ohm. SCSI communication bus includes a bus controller on a single expansion board that plugs into the host computing system. The expansion board is called a Bus Controller Card (BCC), SCSI host adapter, or SCSI controller card.
In some embodiments, single SCSI host adapters are available with two controllers that support up to 30 peripherals. SCSI host adapters can connect to an enclosure housing multiple devices. In mid to high-end markets, the enclosure may have multiple controller interface or controller cards forming connection paths from the host adapter to SCSI buses resident in the enclosure. Controller cards can also supply bus isolation, configuration, addressing, bus reset, and fault detection operations for the enclosure.
One or more controller cards may be inserted or removed from the backplane while data communication is in process, a characteristic termed “hot plugging.”
Single-ended and high voltage differential (HVD) SCSI interfaces have known performance trade-offs. Single ended SCSI devices are less expensive to manufacture. Differential SCSI devices communicate over longer cables and are less susceptible to external noise influences. HVD SCSI is more expensive. Differential (HVD) systems use 64 milliamp drivers that draw too much current to enable driving the bus with a single chip. Single ended SCSI uses 48 milliamp drivers, allowing single chip implementations. High cost and low availability of differential SCSI devices has created a market for devices that convert single ended SCSI to differential SCSI so that both device types coexist on the same bus. Differential SCSI in combination with a single ended alternative is inherently incompatible and has reached limits of physical reliability in transfer rates, although flexibility of the SCSI protocol allows much faster communication implementations.